The present invention relates to a device for the modulated braking of the thread for devices that feed weft to looms or textile machines.
As is known, weft feeders are devices that are suitable to accumulate a reserve of thread in the form of turns wound around a fixed drum and to feed the loom by unwinding the accumulated turns by an amount equal to the length of thread required by the loom at each beating; this length is equal to the transverse dimension, or height, of the fabric being formed. During its motion for unwinding from the drum, the thread is subjected to the action of a braking means which is also associated with the weft feeder; the braking means has the purpose of keeping said thread at a preset mechanical tension that is suitable to ensure its correct unwinding.
During the beating, the thread advancement speed varies in an approximately sinusoidal manner, and the action of the braking means, which depends on the friction coefficient between the thread and the braking means, varies accordingly. Consequently, the tension of the thread is subject to considerable and undesirable increases that cause frequent breakage.
In order to avoid this severe drawback, it has already been proposed to positively modulate the action of the braking means, adapting it to the instantaneous values of the advancement speed or to the mechanical tension of the thread.
Electromagnetically operated modulated-braking devices have already been produced for this purpose; in these devices, the braking means is rigidly coupled to a moving element that supports an energization coil immersed in the field of a permanent magnet. The energization coil is supplied with a current that varies in proportion to the thread advancement rate or to its mechanical tension, and this energization current, by interacting with the field of the permanent magnet, modulates the action of the braking means in order to keep the tension of said thread substantially constant.
A typical modulated-action braking means of conventional devices is constituted by a rigid body which is pressed with variable force against the drum, or against a portion of the drum, of the feeder; the thread runs between said body and said drum or drum portion.
This known solution has some drawbacks. In fact, on one hand it considerably complicates the structure of the feeder, because it is necessary to have an adjustable and flexible support for the braking body, and on the other hand the rigid body that forms the braking means produces undesirable twisting of the thread. Furthermore, if said rigid braking body has a circumference that is equal, or substantially equal, to the circumference of the drum of the feeder, its presence makes it troublesome to use additional braking elements of the known brush type; however, this use is advantageous to avoid--as it is well-known to the expert in the field--the formation of the so-called "balloon".
On the other hand, if the circumference of the rigid braking body is smaller, or much smaller, than the circumference of the drum, in order to facilitate the installation of said additional brush-like braking element, the effectiveness of said small-circumference rigid body is very low in terms of modulating the braking action.
Another known solution that tends to avoid the use of said braking body uses a winder that is located downstream of the drum of the feeder and comprises a series of equidistant fixed rods and a series of equidistant movable rods that are interleaved with the fixed rods and are supported by a movable element that has an energization coil subjected to the field of a permanent magnet. The thread runs between the fixed and movable rods and winds partially around them, forming winding angles that vary according to the movements of the movable rods with respect to the fixed rods produced by corresponding movements of said movable element.
The mechanical tension T.sub.u of the thread at the output of the device is in direct proportion to said winding angles according to the relation EQU T.sub.u =T.sub.i .times.e.sup.n-lf.alpha.
where T.sub.i is the input tension, f is the friction coefficient between the thread and the rods, .alpha. is the winding angle and n is the total number of rods.
The drawback of this further known type of braking means that uses a winder resides in that it essentially operates like a modulating amplifier for the input tension T.sub.i, so that it is necessary to provide an additional braking means that produces said input tension, possibly in a controlled manner; in terms of the modulation produced by the winder-based braking means, it is preferable for the input tension to be as independent as possible of the acceleration imparted to the thread.